The objective of the present proposal is to elucidate the ultrastructural and functional features of the mitotic apparatus (MA) in dividing cells. Experiments are designed to gain further insight into the structure and assembly of the MA and its function in the control of chromosome segregation in normal and neoplastic cells. The materials to be investigated include cultured mammalian fibroblast, HeLa cells, human lymphocytes from normal and leukemic patients, avian fibroblasts, insect spermatocytes, and various mammalian cell hybrids. The cell type to be used in each experiment will be selected to facilitate ease and simplicity of the experimental design. The areas to be investigated include (a) further studies of the ultrastructure and function of the kinetochore, (b) further studies of the centriole and its role in the assembly of the MA and chromosome segregation, (c) relationship between the components of the nuclear envelope and the kinetochore, (d) further studies of the effects of drugs and physical agents on the MA, and (e) mitotic events in mammalian cell hybrids.